The present invention relates to a modulator in an optical transmission module and more particularly to a Mach-Zehnder optical modulator.
Heretofore, as a modulator for an optical transmission module there have been known an electric absorption (EA) type and a Mach-Zehnder interferometer type. The electric absorption type modulator utilizes a mechanism such that if a modulation signal voltage is applied to light propagated through a waveguide, the resulting electric field causes an electric absorption coefficient in a medium to change, thereby intercepting the light. On the other hand, the Mach-Zehnder interferometer type utilizes a mechanism such that when light propagated through a waveguide is branched in two directions and a modulation signal current is flowed through the center of each branch, there occur magnetic fields of opposite phases with respect to grounds provided on opposite sides in a sandwiching relation to the waveguides, so that the phases of light signals propagated through the respective routes become opposite to each other and the phase lead and lag are offset each other when both lights are later combined together. It is the Mach-Zehnder modulator that utilizes this mechanism.
FIGS. 10 to 12 are schematic diagrams showing a schematic construction of the conventional Mach-Zehnder modulator. In these figures, light guided through an optical waveguide 101 is branched to two optical waveguides 117a and 117b at an optical waveguide branch point 102 and the light signals propagated respectively through the optical waveguides 117a and 117b are changed in phase by electric fields developed between an electrode 104 and grounds 105a, 105b in accordance with a modulation signal generated by a modulation signal generator 103. FIG. 11 shows a case where the modulation signal generator 103 is OFF, while FIG. 12 shows a case where the modulation signal generator 103 is ON.
With the modulation signal generator 103 OFF as in FIG. 11, the light signals propagated through the branched waveguides are joined at a junction 106 without any change like their branching at the branch point 102, so that there is no change in the light signal before and after the modulator. On the other hand, when the modulation signal generator 103 is ON as in FIG. 12, refractive indices in the waveguides are changed by electric fields generated respectively between the electrode 104 and the ground 105a and between the electrode 104 and the ground 105b (Pockels effect), resulting in that the light signals propagated through the waveguides 117a and 117b become xc2x190xc2x0 out of phase with each other and become 180xc2x0 out of phase at the junction 106. This can offset the phase lead and lag. Thus, a modulation signal can be imparted to light. In this single Mach-Zehnder modulator, however, the driver""s speed is an upper limit of the transmission speed.
JP-A-112688/1998 discloses a duobinary signal generating method wherein a semiconductor laser, a light intensity modulator and an optical phase modulator are cascaded and one branched signal is subjected to light intensity modulation, while the other signal is subjected to optical phase modulation.
Above-described conventional methods are unable to attain a higher transmission speed than a driver signal which drives the modulator. Besides, since it is only whether light passes or is cut off that is available as a working mode, it has been impossible, either, to set multi-value amplitudes. Therefore, for example in such a high-speed signal propagation as exceeds 40 Gbps, a restriction has heretofore been placed by a driver circuit or by a line which is for propagation of the signal to a modulator, independently of characteristics of a laser diode (LD) and an optical fiber.
In the JP-A-112688/1998, light is phase-modulated and therefore, in case of transmitting light with optical fiber over along distance, it is difficult to reproduce a light signal accurately because the phase changes.
It is an object of the present invention to solve the above-mentioned problems and provide a Mach-Zehnder optical modulator wherein conventional driver circuit and transmission line left intact, a modulator is used like an optical MUX (multiplexer) and a signal corresponding to an integer multiple of a signal generated by a single driver circuit is used to permit an increase in the amount of information capable of being transmitted during a certain period of time.
It is another object of the present invention to provide a Mach-Zehnder optical modulator wherein the amount of information capable of being transmitted can be increased by multi-value amplitudes although the transmission speed remains the same.
For achieving the objects, according to one aspect of the invention, there is provided a Mach-Zehnder optical modulator comprising: a first modulation signal generator for generating a first modulation signal to modulate light signals guided by first and second optical waveguides, the modulation being conducted so that the phases of the light signals become opposite to each other; and a second modulation signal generator for generating a second modulation signal to modulate light signals guided by third and fourth optical waveguides, the modulation being conducted so that the phases of the light signals become opposite to each other; wherein the transmission speed or the amount of information transmitted is increased in accordance with modulation signals provided from the first and second modulation signal generators.
In the aspect of the invention, there is provided means for generating a phase shift between the first and second modulation signals generated from the first and second modulation signal generators respectively, a phase between light signals modulated respectively in the first and second optical waveguides and in the third and fourth optical waveguides is changed, and thereafter both light signals are combined together and outputted.
There is also provided means for generating a phase shift between first and second modulation signals generated from the first and second modulation signal generators respectively, and the third optical waveguide is a joined optical waveguide of both first and second optical waveguides.
Further, the quantity of light passing through the first and second optical waveguides and the quantity of light passing through the third and fourth optical waveguides are made different from each other and the first to fourth optical waveguides are joined together to provide an output.
According to an another aspect of the invention, there is provided a Mach-Zehnder optical modulator which modulates a light signal propagated through an optical waveguide in accordance with a modulation voltage applied, the optical waveguide having plural, at least two, branches, and which comprises plural, at least two, Mach-Zehnder optical modulator portions for modulating light signals propagated through optical waveguides in accordance with the modulation voltage.
Other and further objects, features and advantages of the invention will appear more fully from the following description.